docs: fix python sample spec URLs

[FanouZeng-TT]

Description

The Python sample README's discovery profile example pointed several spec
URLs at paths that do not exist on the published UCP site:

"spec": "https://ucp.dev/2026-04-08/specification/shopping/checkout"

The sample server's actual discovery profile uses the reachable paths without
the extra shopping segment (and overview for the service entry). The old
example URLs return 404, while the updated URLs return 200.

Fix: align the README discovery profile example with the sample server's
runtime profile and the published spec URLs.

Category (Required)

• Core Protocol: Changes to the core UCP specification. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance, contribution process, or project docs. (Requires Governance Council approval)
• Capability: Changes to existing or new capabilities. (Requires Maintainer approval)
• Documentation: General documentation updates. (Requires Maintainer approval)
• Infrastructure: CI/CD, build, and repository infrastructure. (Requires DevOps Maintainer approval)
• Maintenance: Dependency updates, cleanup, and non-functional changes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)
• Community Health (.github): Organization-level templates and community health files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

• uvx pre-commit run --all-files
• git diff --check
• Verified the updated https://ucp.dev/2026-04-08/specification/{overview,checkout,discount,fulfillment} URLs return 200.